A mobile communication system to utilize an IMS (IP Multimedia Subsystem) is defined in non-patent literature 1. In this mobile communication system, upon receiving a call connection request signal (for example, an INVITE signal of SIP) for a mobile terminal UE, the IMS transfers the received call connection request signal to the mobile terminal UE, via an IMS call control signal bearer which the mobile terminal UE establishes in the packet network (for example, Evolved Packet Core).